Processing of consolidated high-purity glass preforms into core cane (otherwise referred to as a blank) is well known in the art. A glass soot, which may include suitable doping, is deposited, for example, by a flame hydrolysis process upon a rotating substrate such as an alumina mandrel. Various methods of flame hydrolysis are described in U.S. Pat. Nos. 3,737,292, 3,823,995 and 3,884,550. The core portion of the soot preform is formed by introducing various gasses in predetermined amounts into a burner flame. This introduction produces oxides that include, for example silicon oxide and germanium oxide. These oxides deposit on the rotating mandrel until the appropriate diameter of the core portion is reached. The oxides may be introduced in various percentages, as desired, to produce various core refractive index profiles as described, for example, in U.S. Pat. No. 3,823,995. The core portion, once formed, is then generally overclad with SiO.sub.2 until a final soot preform diameter is reached. As is well understood, the overclad portion, once consolidated, exhibits a refractive index lower than that of the core. Alternately, the refractive index differences may be achieved by down dope cladding.
Upon completion of the flame hydrolysis process, preferably by an Outside Vapor Deposition (OVD) process, the mandrel with deposited soot is removed from the OVD lathe. Typically, a handle portion is included on the preform and is integral therewith. The mandrel is then removed from the preform thereby leaving a soot preform having an aperture extending along its axial length and positioned at the preform's centerline. The aperture then has a silica plug-like member inserted at its lower end. Subsequently, the preform is inserted into and held in a consolidation furnace. First, chlorine gas is included within the muffle portion of the furnace to aid in water removal from the preform. In particular, chlorine permeates the interstices of the soot preform and flushes out any OH, H.sub.2 or H.sub.2 O contained therein. The preform is then heated at a high temperature (generally in the range of between about 1450.degree. C. to about 1600.degree. C., depending upon preform composition) until the deposited soot consolidates and transforms into a solid, high-purity glass having superior optical properties. Typically, the preform is subjected to gradient consolidation, a technique taught in U.S. Pat. No. 3,933,454 whereby the bottom tip is consolidated first; the consolidation continuing up the preform until completed. It should be recognized that during consolidation, the silica plug-like member combines with and completely seals the lower end of the preform.
As is known to those of ordinary skill in the art, any OH, H.sub.2 or H.sub.2 O included in the consolidated preform or the intermediate core cane may degrade the optical properties of the resultant optical fiber produced therefrom. Reductions of even small amounts of retained OH, H.sub.2 or H.sub.2 O can have substantial benefits in terms of dB/km losses (attenuation) in the resulting optical fiber produced. OH, H.sub.2 or H.sub.2 O content along the centerline aperture is particularly problematic because, for example, in most optical fibers, the maximum field strength of the optical signal occurs at or near the centerline. Therefore, reductions of OH, H.sub.2 or H.sub.2 O present in the preform results in reduced system cost to the end optical fiber user because optical componentry, such as regenerators, amplifiers and the like can be spaced further apart. Therefore, removal of OH, H.sub.2 or H.sub.2 O is a significant problem in optical fibers.
Once the preform is consolidated, it is removed from the furnace and transferred to an argon-filled holding vessel. Next, the preform is drawn, under a vacuum, to close the centerline aperture and stretch the preform into a core cane of constant diameter as is known to those of skill in the art. The core cane is then cut into segments, each of which is then again overclad with SiO.sub.2 soot to an appropriate diameter and again consolidated thereby resulting in a preform which is apertureless. The resulting preform is then transferred to a drawing furnace to draw the optical fiber.
One of the problems encountered during the removal of the preform from the core consolidation furnace is re-wetting of the centerline portion, i.e., the aperture. "Re-wetting" as referred to herein means that OH, H.sub.2 or H.sub.2 O is re-dispersed, diffused, or otherwise deposited on or into the consolidated glass. The mechanism of re-wetting is accomplished as the preform is removed from the furnace. Air replaces the gas present in the aperture due to buoyancy and temperature gradient effects. Because of the humidity present in the air, OH, H.sub.2 or H.sub.2 O re-disperse, diffuse, or otherwise deposit on or into the consolidated preform, and most problematically at its centerline. In particular considerable effort has been spent on methods of reducing re-wetting in glass preforms as any resulting improvement in attenuation translates into lower system cost to the end-user. These efforts, although successful, have resulted in additional steps and expense.
Thus, there is a need for a simple and cost-effective method that reduces the amount of re-wetting of the consolidated preform subsequent to consolidation.